Recent requirements for “green” or substantially less toxic rocket propellants, particularly for use on space stations, have resulted in a search for suitable less toxic propellant compositions that function as effectively as available propellants. Solid rocket propellants are fast burning solids which operate only one time and are not usable in, for example, space station and similar applications where throttle control and on/off switching capability are essential. Liquid propellants provide the throttle and switching control desired for thrust vector control motors. An oxidizer of nitrogen tetroxide with a fuel of hydrazine is an excellent bipropellant combination for this purpose. Hydrazine used as a monopropellant is also attractive for this purpose. Also, catalyzed hydrazine as a monopropellant provides off/on capability. However, these propellants do not conform to the new requirements for environmentally nontoxic propellants because the constituents are extremely toxic or carcinogenic.
Most of the available liquid propellants are bipropellants similar to nitrogen tetroxide and hydrazine discussed above. The liquid oxidizer and the liquid fuel components are stored separately and then mixed when the propellant must be burned. In some cases the ingredients used in bipropellant systems are hypergolic. A hypergolic bipropellant system is one in which the constituents ignite on contact with each other. Although liquid monopropellants are simpler to use than bipropellants, liquid monopropellants that perform as w II as liquid bipropellants have heretofore not been available.
Liquid propellants, many of which have been claimed to have less toxicity, have been disclosed as green propellants, in the prior art. Zletz et al. In U.S. Pat. No. 2,896,407, for example, disclose liquid propellants useful for gas generation and rocket propulsion. The bipropellants disclosed by Zletz et al. require the hypergolic reaction of a liquid fuel and a liquid oxidizer, preferably highly concentrated hydrogen peroxide that may optionally include a dissolved water soluble inorganic salt, such as ammonium nitrate. The hypergolic fuel is an organohalothioborate, such as dimethylchlorodithioborate or its solutions in conventional hydrocarbons. Zletz et al. do not disclose or otherwise suggest premixed monopropellant mixtures or solutions containing water soluble organics, such as alcohols, or water soluble organic salts, such as amine-nitrates, in aqueous hydrogen peroxide solutions. Furthermore, it is not possible to forecast the behavior of four-component mixtures, such as H2O2/H2O/AN/alcohol, based on the properties of three-component mixtures such as H2O2/H2O/AN. Moreover, the hypergolic bipropellant formulations described by Zletz et al. are, by their nature, unsuitable for use as monopropellants.
Rowlinson. U.S. Pat. No. 3,004,842 teaches that foamed solid AN explosives are more sensitive to detonation than either unfoamed or porous beds of granules. He melt-casts compositions containing AN at the melting point of AN, and uses H2O2 as a foaming agent that decomposes to steam and O2 at the casting temperature, forming a solid foam as the temperature is lowered to ambient. His foams are detonable with only a blasting cap and do not require a booster like other AN explosives.
The present invention concerns only liquid solutions and mixtures. Rowlinson uses H2O2 as a foaming agent, not as either a solute or an oxidizer. Also, preferably our monopropellants would be nondetonable.
U.S. Pat. No. 3,470,040 to Tarpley describes inorganic liquid propellant compositions that are essentially unpourable, and thus are gel-like, under storage or shear conditions. These gelled liquid propellants may use a liquid oxidizer, such as red fuming nitric acid and liquid oxygen, and contain ammonium nitrate, have a yield point and flow when pumped. The present invention discloses premixed liquid monopropellant solutions and mixtures and not gels.
Berman, in U.S. Pat. No. 3,143,446, acknowledges the disadvantages of all liquid propellant types of rocket motors and teaches encapsulating reactive liquid oxidizers, including nitrogen tetroxide, or liquid fuels, such as hydrazine, for use in solid propellants. Auxiliary solid oxidizers, such as ammonium nitrate, may be used with the encapsulated liquids. The present invention does not disclose encapsulated propellant ingredients.
Hybrid propellants consisting of a solid fuel, either RDX or HMX, and a liquid oxidizer are taught by Biddle et al. in U.S. Pat. No. 4,527,389. The liquid oxidizer preferred for this purpose is an aqueous solution of hydroxylamine nitrate (HAN) or hydroxylamine perchlorate (HAP). The solid fuel burns by itself to generate fuel-rich combustion products, and the liquid oxidizer is sprayed into the combustion products to oxidize them to completion. Biddle does not disclose premixed liquid monopropellants. Use of hydrogen peroxide as an oxidizer is stated to be unsuitable because it is corrosive. The present invention does not disclose propellants for use in a hybrid rocket motor configuration.
U.S. Pat. No. 5,292,387 to Highsmith et al. discloses ammonium nitrate-containing propellants. These propellants, however, are solid propellants wherein ammonium nitrate is phase-stabilized with a metal dinitramide, preferably potassium dinitramide by dissolving ammonium nitrate and potassium dinitramide in methanol, which is evaporated. It is not suggested that any of these components could be used to form premixed liquid monopropellant solutions and mixtures.
In U.S. Pat. No. 5,837,931, Bruenner et al. disclose solid solutions made of ammonium nitrate, hydrazinium nitrate, hydroxylammonium nitrate and/or lithium nitrate, including eutectics, that are liquid at room temperature and useful as liquid oxidizers for propellants. These propellants, which contain a metal fuel, a hydrocarbon polymer and the liquid oxidizer, form a gel structure that supports the metal fuel. Bruenner et al. does not suggest liquid propellants that do not require the formation of solid solutions or eutectics.
A need exists, therefore, for substantially nontoxic, low detonation sensitivity, environmentally friendly liquid propellants that perform effectively and provide maximum throttle control. A need particularly exists for premixed liquid monopropellant solutions and mixtures with these characteristics.